1. Field of the Invention
The present invention relates to a light receiver which is used for an optical pickup and has a photodiode as an input source and, particularly, to a light receiver with gain switching function.
2. Description of Related Art
An optical pickup is a device for reading and writing data on optical disks such as CD and DVD. The optical pickup typically includes a light receiver which receives reflected light from an optical disk as an optical signal by a photodiode, amplifies an electrical signal generated based on the optical signal and outputs the electrical signal.
Recently, highly functional light receivers are required so as to ensure application to optical pickups with multi-compatibility feature (e.g. CD-ROM, DVD-ROM, DVD-R, DVD-RAM, BD(Blu-ray disk)-ROM). In an optical pickup with multi-compatibility, a light receiver receives optical signals of different intensity levels, such as optical signals for writing/reading, optical signals with different wavelengths, and so on. Still, the light receiver needs to provide an output signal of a constant intensity level.
An optical pickup with multi-compatibility to receive optical signals with different wavelengths and different intensity levels can include a plurality of light receivers with different gains. However, this requires an optical system for inputting an optical signal to each light receiver and thus complicates the optical system of the optical pickup. Further, this hinders the miniaturization of the optical pickup. In addition, preparing a plurality of light receivers results in an increase in the number of components, which increases manufacturing costs.
To address the above drawback, an optical pickup can include a single photodiode and a plurality of amplifiers which have different gains as usage and are connected to the photodiode, so that one amplifier can be selected and activated according to need.
FIG. 3 is a circuit diagram of a light receiver 31 with multi-compatibility according to a related art. A cathode of a photodiode 1 which serves as an input signal source is connected to an input end of a first amplifier 4 and an input end of a second amplifier 3. The first amplifier 4 and the second amplifier 3 are negative-feedback amplifiers which have resistors R1 and R2, respectively, between the input and output. Outputs of the first amplifier 4 and the second amplifier 3 are input to an output circuit 7 through buffers 5 and 6, respectively. The light receiver 31 also includes a switching circuit 8 which switches to cause one of the first amplifier 4 and the second amplifier 3 to enter operating state and the other to enter non-operating state, alternately. The first amplifier 4 is used for processing DVD-R, and the second amplifier 4 is used for processing DVD-RAM, for example. When processing DVD-RAM, an optical signal reading speed is higher, and optical signal intensity is smaller because reflected light from the optical disk is lower than when processing DVD-R. Accordingly, parameters of circuit elements are set so that the second amplifier 3 has a higher gain and a wider bandwidth than the first amplifier 4.
In the light receiver 31 of FIG. 3, the first amplifier 4 and the second amplifier 3 are connected to the photodiode 1. The amplifier in the non-operating state affects the amplifier in the operating state as a load of resistance and capacitance, which leads to an increase in noise to cause an optical signal reading error. Particularly, the second amplifier 3 has a high gain and is largely affected by the first amplifier 4 as a load, and an optical signal reading error is likely to occur during operation of the second amplifier 3. Related arts are described in Japanese Unexamined Patent Application Publication Nos. 2003-22537 and 2005-32282.